Continuous preparation of dimethylvinylcarbinol by isomerizing prenol

Abstract

A process is described for the continuous preparation of dimethylvinylcarbinol (DMVC) by continuously reacting prenol with the aqueous solution of a protic acid in the still or in the lower part of a rectification column while distilling off the DMVC formed in the form of an azeotropic mixture with water, which comprises a) during the reaction ensuring that as far as possible no liquid two-phase mixture forms in the reaction mixture, b) distilling off the DMVC which forms at a reflux ratio of at least 2, as a result of which there is virtually no more prenol present in the distillate from the column, and c) setting the reaction volume and/or the amount of prenol added per hour and/or the concentration of protons in the reaction mixture in such a manner that the quotient, defined as residence time concentration coefficient, of the reaction volume and the volume of prenol added per hour multiplied by the concentration of protons in the reaction mixture has a value between 0.001 h·mol/l and 1 h·mol/l. In this connection, the formation of a liquid two-phase mixture in the reaction mixture is prevented, in addition to avoiding too large a prenol excess, by continuously taking from the reaction mixture a partial stream, passing it over a phase separation vessel, completely or at least largely removing there the small quantities, formed in the course of the reaction, of organic by-products which are insoluble or only partially soluble in water, and returning the aqueous phase to the reaction volume.

We claim:  
     
       1. A process for the continuous preparation of dimethylvinylcarbinol (DMVC) by continuously reacting prenol with the aqueous solution of a protic acid in the still or in the lower part of a rectification column while distilling off the DMVC formed in the form of an azeotropic mixture with water, which comprises 
       a) during the reaction ensuring that as far as possible no liquid two-phase mixture forms in the reaction mixture,  
       b) distilling off the DMVC which forms at a reflux ratio of at least 2 and  
       c) setting the reaction volume and/or the amount of prenol added per hour and/or the concentration of protons in the reaction mixture in such a manner that the quotient, defined as residence time concentration coefficient, of the reaction volume and the volume of prenol added per hour multiplied by the concentration of protons in the reaction mixture has a value between 0.001 h·mol/l and 1 h·mol/l.  
     
     
       2. A process for the continuous preparation of dimethylvinylcarbinol as claimed in claim  1 , which comprises, in order to prevent the formation of a liquid two-phase mixture in the reaction mixture, continuously taking from the latter a partial stream, passing it over a phase separation vessel, completely or at least largely removing there the small quantities, formed in the course of the reaction, of organic by-products which are insoluble or only partially soluble in water, and returning the aqueous phase to the reaction volume. 
     
     
       3. A process as claimed in claim  1 , which comprises setting the reaction volume and/or the amount of prenol added per hour and/or the concentration of protons in the reaction mixture in such a manner that the quotient, defined as residence time concentration coefficient, of the reaction volume and the volume of prenol added per hour multiplied by the concentration of protons in the reaction mixture has a value between 0.01 h·mol/l and 0.15 h·mol/l. 
     
     
       4. A process as claimed in claim  1 , which comprises distilling of the DMVC at a reflux ratio of from 2 to 25. 
     
     
       5. A process as claimed in claim  1 , which comprises bringing the distillate obtained at the head of the column into close contact with an extractant which is only partially miscible with water, separating the organic phase which forms from the aqueous phase and returning the aqueous phase to the reaction volume. 
     
     
       6. A process as claimed in claim  5 , which comprises separating the organic phase obtained at the head of the column in a rectification column, a mixture of a little DMVC, water and the extractant being obtained at the head thereof and pure DMVC being obtained from the still. 
     
     
       7. A process as claimed in claim  6 , which comprises using the mixture of a little DMVC, a little water and the extractant, which is produced at the head of the rectification column, for extracting the distillate obtained at the head of the isomerization column. 
     
     
       8. A process as claimed in claim  1 , which comprises carrying out the isomerization in a column having from 3 to 55 theoretical plates. 
     
     
       9. A process as claimed in claim  1 , which comprises adjusting the pH of the aqueous solution of protic acid to from 1.2 to 3.5. 
     
     
       10. A process as claimed in claim  1 , which comprises condensing the vapors leaving the head of the isomerization column and the vapors leaving the head of the rectification column in a common condenser. 
     
     
       11. A process as claimed in claim  4 , wherein said reflux ratio is from 4 to 20.